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British Established 1907; incorporating 'The Zoologist', established 1843

The strange case of the Whistling Oofoo What are runt ? Humphrey Q. P. Crick

Every now and then, birdwatchers come across strange miniature eggs in . Often perfect replicas of normal eggs, these 'runt', 'dwarf, 'cock', 'witch' or 'wind' eggs (Romanoff & Romanoff 1949) have attracted attention and have been regarded with some superstition. In the earlier part of this century and before -collecting was made illegal, schoolboy egg-collectors recorded these eggs as coming from strange species such as the 'Whistling Oofoo' (B. Harrup in lift.). Just what are these eggs, and why do they occur? A Dictionary of Birds (Campbell & Lack 1985) suggests that runt eggs are the last laid in a and occur because the female becomes exhausted. At Nor­ wegian colonies where the eggs of Herring Larus argentatus and Great Black-backed Gulls L. marinus are collected for human consumption, runt eggs are thought to be laid by females exhausted from re-laying after repeated robbery (R. T. Barrett in litt.). It is, however, quite normal for eggs to vary in size and shape in relation to such factors as position in the laying sequence, age of the female, time through the breeding season, and the weamer, and it may be that runts are simply part of the normal continuum of egg-size variation.

[Brit. Birds 88: 169-180, April 1995] 169 170 What are runt eggs? In the case of domestic poultry, it has been suggested that runt eggs are abnormal accidents, and that they are laid by active hens and not by immature birds (Pearl & Curtis 1916, cited in Koenig 1980b), although Romanoff & Romanoff (1949) contradicted this by stating that they are laid more often by immatures than by adults. Harrison (1951) reported that runt eggs are 'the result of egg formation around a foreign body of some kind, such as a small blood clot, or a piece of inspissated albumen', but he did not consider the ecological circumstances in which such eggs are produced.

New records of runt eggs In 1989, I was sent a photograph of the runt egg of a Blackbird Turdus merula by P. F. Hollins, a participant in the BTO's Record Scheme, who was greatly puzzled by the egg. Following publication of this photograph, together with a brief note and a request for further observations (Crick 1989), I received a considerable number of replies. Some of these contained detailed records, and they are briefly reviewed below in relation to previously published information, in an attempt to shed more light on the phenomenon of the 'Whistling Oofoo'. A total of 87 instances of runt eggs was recorded, involving 44 species, ranging from divers Gavia to buntings Emberiza (table 1). They were fairly likely to occur in reduced or incomplete clutches (22% of records: for Eurasian Curlew Numenius arquata, Barn Hirundo rustica, Blackbird, Reed Warbler Acrocephalus scirpaceus, Blue Tit Parus caeruleus, Great Tit P. major, Carrion Crow Corvus corone and Bullfinch Pyrrhula pyrrhuh). In the majority of cases, only one runt egg was found in a clutch of otherwise normal eggs. Measurements of runt eggs are given in table 2, and can be compared with similar measurements from the literature shown in table 3. (Note that, throughout this paper, 'size' is used to indicate relative volume, calculated from length X breadth X breadth.) Koenig (1980a) found that runt eggs could be recognised as those which were less than 75% of the size of the rest of the eggs in the clutch. In both tables, all runt eggs are less than 75% of the average egg size for each species, and ranged from 12% to 71% of the normal. From the information in tables 2 & 3 combined, the average runt egg reported among non- was 30% of normal volume (n = 12) and for passerines was 38% of the normal (n = 26), but the difference was not statisti­ cally significant.1 When the relative volume of a runt egg (the average for each species) was plotted against the volume of a normal egg, however, a significant relationship was found, species with larger eggs tending to produce relatively smaller runt eggs.2 Since non-passerines are usually larger than passerines, they will tend to lay relatively smaller runt eggs. Runt eggs usually lack a full yolk (Koenig 1980a; M'Williams 1927), although 65% contain a particle of yolk that is probably the nucleus around which the egg forms (Romanoff & Romanoff 1949). Of the previously unpublished records (table 1), none was reported as having a yolk, while a definite lack of yolk was reported for runt eggs of Lesser Black-backed Gull Lams fuscus, Barn Swallow,

1 Superscript numbers 1, 2 and 3 refer to results of statistical tests, which are shown in the Appendix on page 180. British Birds, vol. 88, no. 4, April 1995 171 Hedge Accentor Prunella modularis, Blackbird and Reed Warbler. Hatchability of small eggs is often lower than that of bigger eggs (Rofstad & Sandvik 1985), and, as might be expected from a lack of yolk, none of the runt eggs in table 1 was reported to have hatched. Table 1. Previously unpublished records of runt eggs. The composition of each clutch, where known, is given as number of runts + number of normal eggs. No. of Clutch composition Species records (where known)

Red-throated Diver Gavia stellata 1 Northern Gannet Mows bassanus 1 Shag Phalacrocorax aristotelis 1 Mute Swan Cygnus olor 2 1 + 7,1 + 8 Goose Branta canadensis 1 1+5 Common Eider Somateria mollissima 1 1+4 Hen Harrier Circus cyaneus 1 Buteo buteo 2 1 + 1,1+2 Red Lagopus lagopus 1 1+9 Moorhen Gallinula chloropus 2 Common Coot Fulica atra 2 1 + 5 Oystercatcher Haematopus ostralegus 4 1+2,1+3,1+2,2 + 2 Great Ringed Plover Charadrius hiaticula 1 1+3 Northern Lapwing Vanellus vanellus 3 1+4,1 + 1,1+3 Eurasian Curlew Numenius arquata 2 1+0,1+4 Common Greenshank Tringa nebularia 1 Black-headed Gull Larus ridibundus 1 1 + 1 Lesser Black-backed Gull L fuscus 1 2+1 Herring Gull L argentatus 3 Great Black-backed Gull L marinus 3 1+3,1+2 Common Tern Sterna hirundo 2 Razorbill Aha torda 1 Sky Lark Alauda arvensis 1 Barn Swallow Hirundo rustica 2 1 + 1,1+4 Yellow Wagtail Motacilla flava 1 1+4 Grey Wagtail M. cinerea 2 Pied Wagtail M. alba 1 1+4 Dipper Cinclus cinclus 2 1+4 Hedge Accentor Prunella modularis 1 Robin Erithacus rubecula 1 Whinchat Saxicola rubetra 1 2 + 3 Blackbird Turdus merula 9 1 + 2,1 + 3 (three), 1 + 4 (two) Song T.philomelos 2 1 + 4 (two) Reed Warbler Acrocephalus scirpaceus 2 1 + 2,1 + 0 Pied Flycatcher Ficedula hypoleuca 2 Blue Tit Parus caeruleus 12 1+3,1+4,1+5,1+7,1+8, 1+9,1 + 13,2 + 7,2 + 8,6 + 0, 7 + 0 Great Tit P. major 1 1+3 Magpie Pica pica 1 Corvus frugilegus 1 1+3 Carrion Crow C. corone 3 1 + 3,2 + 0 (two) 'Hooded' Crow C. c. comix 1 Common vulgaris 1 Linnet Carduelis cannabina 1 1+3 Bullfinch Pyrrhula pyrrhula 1 1 + 1 Reed Bunting Emberiza schoeniclus 1 1+3 172 What are runt eggs? The shape and colour of runt eggs are usually normal (Romanoff & Romanoff 1949), although some may be darker in colour (M'Williams 1927). For the eggs in table 1, normal coloration was recorded for one Common Coot Fulica atra, Red Grouse Lagopus lagopus, one Eurasian Curlew, one Song Thrash Turdus phibmelos, one Hedge Accentor, three Blackbirds, Blue Tit and Linnet Carduelis cannabina. Abnormal coloration was recorded for Red-throated Diver Gavia stellata (darker), Black-headed Gull Lams ridibundus (grey), one Northern Lapwing Vanellus vanellus (brown), one Eurasian Curlew (pale green) and one Blue Tit (no coloration). Shape was usually undescribed, except for the following abnormalities: 'cylindrical (one Common Buzzard Buteo buteo, Common Greenshank Tringa nebularia, Razorbill Alca torda, one Blackbird); 'elongate' (Shag Phalacrocorax aristotelis), with one of the runt eggs of Barn Swallow looking like the fusion of two eggs; and 'rounded' (one Oystercatcher Haematopus ostralegus, Black-headed Gull). One Blue Tit clutch contained eggs which were 'misshapen with knobbly protuberances'. Abnormalities in shape are less common than runt eggs (Romanoff & Romanoff 1949). The rates at which runt eggs occur have rarely been documented in the field, but available information is summarised in table 4. Rates vary from one in every 5,000 eggs (gulls) to nearly one in 25 (Acorn Melanerpes formi- civorus). The latter is unusually high and has been investigated in detail by Koenig (1980a,b). The average rate of occurrence among non-passerines, excluding Acorn Woodpecker, is 35 in 10,000 eggs (n = 7 species) and for passerines is 16 in 10,000 (n = 6 species). The samples were too small to detect any statistically significant differences3, but more than half of the non- rates were greater than all but one of the passerine rates. Table 2, Previously unpublished measurements of runt eggs compared with measure­ ments of normal eggs. Normal-egg measurements are from Harrison (1975). Measurements in parentheses are made by the author from photographs, and are used only to calculate relative size of the runt egg compared with normal egg(s) in the same photograph; they are not measurements of real eggs. Relative size of runt egg is calculated using Preston's (1974) equation of (length X breadth X breadth of runt)/(length X breadth X breadth of normal egg). Under Lesser Black-backed Gull, 49% represents the average size of runt eggs from one clutch. LENGTH x BREADTH (mm) Relative size Species Runt egg Normal egg of runt

Common Eider Somateria mollissima (15.3 x 11.1) (29.5 X 19.5) 17% Oystercatcher Haematopus ostralegus 45.0 x 31.5 57.0 x 40.0 49% Great Ringed Plover Charadrius hiaticula (23.0 x 17.1) (12.6 x 9.7) 18% Northern Lapwing Vanellus vanellus (30.0 X 20.5) (45.8 X 31.3) 28% Eurasian Curlew Numenius arquata 37.5 x 28.1 67.6 x 47.9 19% Lesser Black-backed Gull Laws fuscus 56.0 x 36.0 67.6 x 47.0 49% 48.5 x 33.0 35% Sky Lark Alauda arvensis (8.0 x 6.3) (12.7 x 8.9) 32% Barn Swallow Hirundo rustica 12.0 x 10.0 20.0 X 13.7 32% Hedge Accentor Prunella modularis 14,0 X 10.0 19.9 X 14.7 33% Blackbird Turdus merula 18.9 x 15.7 29.4 x 21.7 34% Reed Warbler Acrocephalus scirpaceus 14.0 x 10.0 18.4 X 13.6 41% 13.5 x 11.6 53% Blue Tit Paws caeruleus 13.9 x 10.7 15.6 x 12.0 71% 11.0 X 8.0 31% Great Tit/? major (36.5 X 28.3) (48.2 X 37.8) 42% British Birds, vol. 88, no. 4, April 1995 173

Possible explanations

When do runt eggs appear? Are they associated with energy shortages, or with first or last eggs in a clutch, or with the age of the layer? A number of interesting observations by correspondents threw light upon these questions.

Energy shortage As mentioned above, it has been suggested that runt eggs are laid when females are exhausted or lack resources (see also O'Connor 1984). This could be the underlying influence for several of the possible explanations discussed below, such as adverse weather, immaturity and old age. More generally, energy shortage may be expected to occur more frequently among species for which clutch production represents a large energetic investment. Precocial birds, since they produce eggs with relatively large yolks compared with altricial species, have high costs of egg production. Ricklefs (1974) showed that daily cost of clutch production for precocial non-passerines was three to four times that for altricial passerines and raptors. This may explain why runt eggs appear to be more common among non-passerines than among passerines.

Adverse weather O'Connor (1979) noted that, during periods of bad weather, the second eggs laid by Common Swifts Apus apus are 0.11 g lighter than the first, instead of the normal 0.10 g heavier. House Martins Delichon urbica, too, apparently lay smaller eggs during adverse weather (Campbell & Lack 1985). The egg sizes of Common Redstart Phoenicurus phoenicurus, Pied Flycatcher Ficedula hypoleuca, Great Tit and Common Starling Sturnus vulgaris in northern Finland increase slightly as average air temperature becomes warmer (Ojanen el al. 1981). Du Feu (in litt.) noted that, although a Blue Tit clutch containing two runt eggs (table 1) was associated with poor weather, the other runt eggs he found were in years with seemingly normal weather.

Heritability and the individual Heritability in egg size can be high, at least for the Great Tit (Ojanen et al. 1979). There is, however, no evidence that the production of runt eggs is inher­ ited, at least among domestic hens (Romanoff & Romanoff 1949). There is some evidence that individual birds may lay runt eggs repeatedly. Clutches containing more than one runt were laid by Oystercatcher, Lesser Black-backed Gull, Whinchat Saxicola rubetra, Blue Tit and Carrion Crow (table 1). Previous instances have been recorded for Canada Goose Branta canadensis (Manning & Carter 1977), Tufted Aythya fuligula and Song Thrush (M'Williams 1927), Common Eider Somateria mollissima and Red-backed Shrike Lanius coUurio (Harrison 1951), Black-headed Gull (Walters 1989), Grey Catbird Dumetella carolinensis (Rothstein 1973), Pied Flycatcher (Slater & Jen­ nings 1987), Common Starling (Ricklefs 1975) and Linnet (Jourdain 1925). Zeleny (1983, cited in Mulvihill 1987) reported a colour-ringed Eastern Sialia sialis laying four rant eggs in three separate clutches in one season. In table 1, the two full clutches of misshapen Blue Tit eggs occurred in successive 174 What are runt eggs? years in the same , while clutches of a pair of runt eggs of Carrion Crow were laid in the same nest in successive years: these two instances suggest that, in each respective case, the same individual birds were responsible.

Age Young birds often lay relatively small eggs (O'Connor 1984), as in the case of Wandering Albatross Diomedea exulans (Croxall et al. 1992), Shag (Coulson et al. 1969), Tufted Duck (Hill 1984), House Wren Troglodytes aedon (Kendeigh et al 1956), Eurasian Jackdaw Corvus monedula (Soler 1988) and domestic (Romanoff & Romanoff 1949). Other studies, however, have found female age to have little effect on egg size of, for example, Great Tit and Pied Flycatcher (Ojanen et al. 1979) and Common Eider (Baillie & Milne 1982). Among the records listed in table 1, parental age was reported for two female Blue Tits: both were hatched in the previous year and were breeding for the first time, although, given the low survival rates of Blue Tits, this is not particularly meaningful. It is also possible that old age and senescent deterioration in reproductive capability could play a part in the production of runt eggs. Harrison (1951) described a case of a Mallard Anas platyrhynchos that laid 82 normal eggs during 1928-32, but then produced 14 runt eggs in 1933. One of the Mute Swan Cygnus olor records in table 1 concerned a ten-year-old female, but the other involved a three-year-old bird.

Position in the laying sequence Studies of variation in egg size through the laying sequence show that size can increase or decrease, or show no trend, although a trend of increase is most common (Ojanen et al. 1981; Slagsvold el al. 1984; Rofstad & Sandvik 1985). With species that start incubating before the clutch is complete, the eggs hatch asynchronously, producing a hierarchy of chick size within the brood. This may be a mechanism to allow brood reduction during times of food shortage, with the smallest and youngest chick dying first (Lack 1947; Magrath 1990). A small last egg may be a means by which differences in chick size can occur without asynchronous hatching (Rofstad & Sandvik 1985), and tends to occur among altricial birds (Slagsvold et al. 1984). Among altricial species with asynchronous hatching, a smaller last egg may hatch relatively quickly, thereby reducing the difference in chick size within a brood (Parsons 1972). The converse is that a large last egg is a means of reducing the size difference between the last chick and its older siblings, tending to occur among precocial birds (Slagsvold et al. 1984); this has been recorded for the Shag (Coulson et al. 1969). There is also

Plates 40-47. Facing page, runt eggs within clutches of normal eggs. UiFr-HAND PHOTOGRAPHS, top to bottom: Red-throated Diver Gavia stellata, , June 1989 (Graham Bundy); Common Eider Somateria moUissima, Sweden, June 1951 {Viking Olsson); Great Ringed Plover Charadrius hiaticula, Merseyside, April 1986 (Jack Taylor); Northern Lapwing Vanellus vanellus, Leicestershire, April 1983 (Martin B. Withers) RIGHT-HAND PHOTOGRAPHS, top to bottom: Blackbird Turdus merula, Essex, summer 1989 (P. F. Hollins); Song Thrush T. philomelos, Sweden, May 1965 (Viking Olsson); Great Tit Paras major, Sweden, June 1988 (Viking Olsson); Reed Bunting Emberiza schoeniclus, West Midlands July 1990 (J. W. Saunders)

176 What are rant eggs? the extreme case of the crested penguins Eudyptes that lay two eggs, the second of which is habitually 20-70% smaller than the first (Lamey 1990). Although the larger egg almost always produces the sole surviving fledgling, the adaptive sig­ nificance of this phenomenon is still poorly understood; this characteristic of crested penguins suggests, however, that the forming of runt eggs may, under circumstances as yet unknown, be adaptive. Alternatively, small last eggs may provide a target for predators, directing preda- tion away from the larger, more valuable eggs in a clutch. The third chicks of Glaucous-winged Gulls Lams ghucescens are relatively unlikely to fledge, and so the third and smaller egg is relatively expendable. These third eggs are not only smaller but also paler than other eggs of this species and, indeed, are preferentially preyed on by other Glaucous-winged Gulls and by Northwestern Crows Corvus caurinus (Verbeek 1988). Smaller third eggs among gulls may also be a result of decreased courtship-feeding of the female by the male (Quinn & Morris 1986). Runt eggs have been found to appear at any stage in the laying sequence (M'Williams 1927; Harrison 1951; Walters 1989), and this was confirmed in the present analysis. Those listed in table 1 were recorded as: first in a clutch of seven and among the first two eggs in a clutch of six (Blue Tit); first and second in a clutch of three (Common Tern Sterna hinindo) (Perry 1990); among the first three of a clutch of four (Great Ringed Plover Charadrius hiaticuld); the sixth egg in a clutch of eight (Mute Swan), having been laid after an interval of only one day instead of the usual two; among the first seven of a clutch of 14, among the first four of a clutch of nine, and one among the first four and another among the last five within one clutch (all Blue Tits); among the last three in a clutch of five (Yellow Wagtail Motacilla flava); and last eggs in clutches of nine (Mute Swan), of three and four (Great Black-backed Gull), of six (Canada Goose), of four (Blue Tit) and of five (Northern Lapwing).

Seasonal changes As with laying sequence, egg size can tend to increase, decrease or remain the same through the nesting season (Rofstad & Sandvik 1985), perhaps related to availability of food (Bancroft 1984). The size of Great Tit eggs tends to increase between first and replacement clutches, depending on time taken to re-lay (Ojanen et al. 1979). Apparent seasonal size decreases may occur as a result of late laying by young birds that form small eggs (e.g. Soler 1988), but such decreases can also be found within age-classes (Coulson et al. 1969). Runt eggs are apparently 'not uncommon' among Common Eiders at the start of the laying season (Harrison 1951). Although approximate laying dates were provided for about half the records in table 1, none, however, was unusually early or late.

Social factors Koenig (1980b) found an unusually high incidence of runt eggs among Acorn , the occurrence being four times greater at nests in which two females, rather than one, laid. He suggested that neither communal nesting nor hole-nesting alone, but instead a combination of the two, might explain the phe­ nomenon, perhaps caused by competition at the nest hole between two females. British Birds, vol. 88, no. 4, April 1995 177

Table 3. Previously published measurements of runt eggs in comparison with measurements of normal eggs. Normal-egg measurements are from Harrison (1975) or, for Nearctic species, from the listed reference. Relative size of runt eggs is calculated as in table 2. Average size of multiple runt eggs found in one clutch is also given. Under Linnet, measurements of the second runt egg are the average of three eggs in a clutch of four. Relative LENGTH X BREADTH (mm) size Species Runt egg Normal egg of runt Reference

Canada Goose 46.4 X 35.8 85.7 X 58.2 20% Manning & Branta canadensis 61.1 x 35.2 24% Carter 1977 56.0 X 34.3 S} Semipalmated Sandpiper 22.1 x 16.1 29.8 X 21.5 42% Mannings Calidris pusilla Carter 1977 Black-headed Gull 33.5 X 25.7 51.9 x 37.2 31% Walters 1989 Lams ridibundus Common Tern 24.0 x 21.0 41.3 X 30.5 »} 28% Perry 1990 Sterna hirundo 24.0 X 21.0 Tawny 39.0 X 29.5 46.7 X 39.1 48% Barta 1989 Strix aluco Grey Catbird 20.5 X 15.2 23.3 X 17.5 g} 64% Rothstein 1973 Dumetella carolinensis 19.6 X 15.2 11.0 X 9.0 20.7 X 16.3 16% Mulvihill 1987 Sialia sialis Blackbird 22.0 X 17.0 29.4 x 21.7 46% Driver 1982 Turdus merula Song Thrush 15.0 X 12.0 27.4 X 20.8 18% Ticehurst 1925 T, philomelos 20.5 X 17.1 51% 1 52% 21.5 x 17.1 53% r 22.0 X 17.8 59% 22.5 X 17.9 61% I 22.5 x 18.2 63% } 62% 23.4 X 18.0 64% J Pied Flycatcher 12.8 x 10.6 17.9 X 13.4 45% Slater & Ficedula hypoleuca Jennings 1987 Common Grackle 18.8 X 13.7 28.5 x 20.9 28% Rickiefs 1975 Quiscalus quiscula 15.9 x 14.0 25% 17.5 x 16.3 37% 22.0 X 17.8 56%

Common Starling 15.1 X 13.3 30.2 X 21.2 20% 1 ?R0/„ Ricklefs 1975 Sturnus vulgaris 19.1 X 15.3 33%]26/° Linnet 11.0 X 8.5 18.2 X 13.2 25% \ MO/. Jourdain 1925 Carduelis cannabina 12.0 x 9.5 34% r 9.0 x 6.5 12% Red-winged Blackbird 17.3 X 13.7 24.8 x 17.6 42% Rothstein 1973 Agelaius phoeniceus 14.2 X 12,6 29% 17.2 X 13.6 41% 178 What are runt eggs? If competitive interactions were a factor, then densely colonial species might be expected to have higher rates of runt-egg production. In table 4, where the majority of non-passerines were colonial and the majority of passerines were solitary nesters, this does appear to be the case. Competition may, therefore, provide an additional factor to explain the different trends between the taxo- nomic groups.

Table 4. Incidence of runt eggs among bird species for which at least 200 eggs have been checked. Numbers of eggs checked are estimated for Red-throated Diver and Shag by multiplying number of nests inspected by normal clutch size; and, for Blue Tit, number of nests inspected times mean clutch size of 9.52 (from 757 nest record cards, 1962-81) No. of No. eggs % Species runt eggs checked runts Source

Red-throated Diver Gavia stellata 1 400 0.25 Table 1 Shag Phalacrocorax aristotelis 2 300 0.67 Table 1 Canada Goose Branta canadensis 3 500 0.60 Table 3 Gulls Laws (4 species) 1 4,560 0.02 Koenig 1980b Black-headed Gull Lams ridibundus 39 107,500 0.04 Walters 1989 Herring Gull L argentatus 2 8,000 0.02 Table 1 Great Black-backed Gull L mar'mus 2 237 0.84 Table 1 Acorn Woodpecker Melanerpes formicivorus 50 1,157 4.32 Koenig 1980b House Wren Troglodytes aedon 2 1,347 0.15 Kendeigh ef al. 1956 Reed Warbler Acrocephalus scirpaceus 2 3,381 0.06 Table 1 Pied Flycatcher Ficedula hypoleuca 23 5,500 0.42 Table 3 Blue Tit Parus caeruleus 3 3,256 0.09 Table 1 4 2,666 0.15 Table 1 Common Grackle Quiscalus quiscula 1 1,277 0.08 Table 3 Common Starling Sturnus vulgaris 2 2,000 0.10 Table 3

Conclusion The production of runt eggs is a rare event within a species, occurring on average once in every 600 eggs of passerines and once in every 300 eggs of non-passerines. It is, however, widespread, having been recorded for over 50 species covering most families. As more species are studied in detail, more instances of runt eggs are sure to be found. This review of the possible factors that may influence the production of runt eggs suggests that bad weather, age, laying sequence and timing within the breeding season are probably unimportant. There appears to be a difference between passerines and non-passerines: the latter seem more likely to produce runt eggs than do passerines and tend to produce relatively smaller ones. This may be due to the greater energetic demands of egg production on non- passerines, associated with the need for precocial or semi-precocial young. Aggressive interactions between birds within densely packed colonies may also be a contributory factor in producing a difference between the two groups. Rothstein (1973) suggested that the rarity of runt eggs implies a strong selec­ tion pressure against their production. Certainly, runt eggs are usually yolkless and do not hatch. If individual birds had a genetic tendency to produce runt eggs, then they would be at a severe selective disadvantage. This may, however, British Birds, vol. 88, no. 4, April 1995 179 be a further factor that could explain the higher incidence among non- passerines: the latter, being generally longer-lived, may have more opportunities to breed than passerines, thereby lessening the effect of the occasional runt egg on their lifetime breeding success.

Acknowledgments

I should like to thank all the correspondents and BTO nest-recorders (NRC = nest record cards) for generously providing the new observations (table 1) that made this paper possible: R. G. Adam, P. Anderson (NRC), M. Baker, R. T. Barrett, K. Briggs, R. H. Bucknall, G. Bundy, M. Calvert, F. R. Cannings, J. Clarke, L. Cordrey (NRC), J. C. A. Craik, P. J. Dare, J. Driver (NRC), C. Du Feu, E. Dunn, D. Eva, D. M. Francis, D. B. Ginn, D. Harrison & J. Oldland (NRC), B. Harrup, P. F. Hollins, D. Holman, D. T. Ireland, W. Kennedy, R. Lambert (NRC), J. M. S. Lewis (NRC), A. Macdonald, H. Mayer-Gross (NRC), C. Mon- caster (NRC), J. P. Moulton, V. Olsson, R. Peart, J. A. L. Roberts (NRC), M. E. H. Robinson, B. Sage, J. W. Saunders, E. L. Sawyer, M. P. & S. Skomer (NRC), K. Spencer, B. Standley, J. Szczur (NRC), J. Taylor, W. Taylor, P. S. Thompson, J. Tomlinson, J. H. Turney, S. Tyler, B. Walker, D. Walker, D. C. Walker, E. H. Webb, M. B. Withers, H. Woodland, B. Zonfrillo. Caroline Dudley was very helpful in bringing observations of runt eggs on nest record cards to my attention. Stephen Baillie, Caroline Dudley, David Glue, Robert Prys-Jones and Michael Walters kindly read and made many useful comments on the manuscript. Susan Waghorn kindly designed the table layout. The Nest Record Scheme forms part of the BTO's Integrated Population Monitoring Pro­ gramme carried out under a contract from the Joint Nature Conservation Committee, on behalf of English Nature, Scottish Natural Heritage and the Countryside Council for Wales, and under a contract from the Department of Environment for .

Summary Runt eggs, which have a volume less than 75% of that of a normal egg, are shown to occur widely among birds, with an incidence of one in 600 passerine eggs and one in 300 non- passerine eggs. A total of 87 new observations is reported for 44 Western Palearctic species. Runt-egg production does not appear to be associated with weather, age, position in the laying sequence or time of year, but individual birds may illustrate a proneness to laying runt eggs. Birds that lay larger eggs tend to produce relatively smaller runt eggs, possibly as a result of the greater energy demands of egg-laying. Runt eggs may be more common in densely colonial species. These last two factors may predispose non-passerines to produce runt eggs more fre­ quently than do passerines.

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Dr Humphrey Q. P. Crick, The British Trust for , The Nunnery, Nunnery Place, Thetford, Norfolk IP24 2PU

Appendix

1. Mann-Whitney z = 1.56; P = 0.12. 2. Relative size (see Preston 1974) of runt egg (%) = 70.2 - (3.5 log (normal egg volume)); F = 4.64; P = 0.04. 3. Mann-Whitney z = 0.25; P = 0.8.